Canon law is the internal law of some Christian sects. The Roman Catholic Church and Eastern Orthodox both have very old and complex legal systems. It is not "law" in the sense that it is legally binding on all citizens, but it does establish a system of rules along with a mechanism for holding trials and imposing punishments that is used within the Church. As a legal system, canon law is very old; the canon of the Roman Catholic Church is one of the oldest continuously operating legal systems in the world.
The term "canon" comes from a Greek word meaning "council." The internal law of the Church was initially developed by councils of religious scholars who wished to establish ground rules that applied to the internal workings of the Church. It covers everything from the activities of religious officiants to the grounds for excommunication, and includes standards for trials and other hearings that may be held to decide on Church matters. Members of the Church as well as officiants are subject to the mandates set out in Church law.
The original canon law was based very much in Roman law. As Christianity began to experience schisms and split into sects, the concept of canon law was taken along into many of these sects. Different sects adapted and changed the law to meet their needs. Numerous societies incorporated Christian law into their legal systems and drafters of canon in turn borrowed from civil and criminal law.
Canon law does not apply to people who are not members of the Church, and individual Christians may follow it to varying degrees. Some Christians have disagreements with certain aspects of the Church law and have pushed for reforms to modernize the law and address emerging ethical and religious issues. Some of these individuals may be involved in activities that are technically grounds for sanctions and excommunication under canon law, and they may use this as an illustration that certain aspects of the law are clearly outdated.
Numerous translations of canon law are available online and in published legal texts. Canon law scholars can study the law from a purely academic sense or because they are interested in applying it. Several educational institutions offer degrees in canon law to people who wish to become canon lawyers. Canon lawyers can represent people in ecclesiastical courts, provide legal advice on interpretations of canon law, and offer other services to Church officials and members. Canon lawyers are not admitted to practice before the bar, only before courts and tribunals held by the Church.